1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus which forms an image by scanning a record medium by a laser beam.
2. Related Background Art
As a high speed image forming apparatus, an apparatus which simultaneously scans a record medium by a plurality of laser beams modulated by respective information signals has been proposed and disclosed in Japanese Patent Publication No. 62-59506 and Japanese Patent Publication No. 63-28304. This apparatus is explained below in conjunction with FIGS. 17 and 18. A light source unit 9 is a semiconductor laser diode array comprising a plurality of laser light sources. Center light beams ha, hb, hc and hd are emitted from light output units 9a, 9b, 9c and 9d of the light source unit 9 in parallel to an optical axis g of a focusing lens 1401. The center light beams pass through the optical axis of the focusing lens 1401 and a cylindrical lens 1402 and reach a deflection mirror plane of a rotating polygon mirror 1403. The light beams reflected by the deflection mirror plane of the rotating polygon mirror 1403 are focused onto a surface of a record medium 15 by an anamofic scan lens system comprising a spherical lens 1404 or a torric lens 1405 to form an image. A reflection mirror 1406 is arranged at an end of the scan line to guide the light beams to a light receiving device 101 (photo detector).
A light shielding plate 110 is arranged in front of the light receiving device 101. The light shielding plate 110 is provided to exactly detect the position of the beam. When the light beam reflected by the reflection mirror 1406 intercepts an edge of the light shielding plate 110, the intercepted light is irradiated to the light receiving device 101 which outputs an electrical signal in accordance with an irradiation intensity.
As shown in FIG. 19, when the laser spots radiated on the record medium have the same light intensity distribution and are at non-right angles with a main scan direction and photo-sensing apertures of the light receiving device 101 have a size to sense all beam spots simultaneously, the light receiving device 101 produces the output signal as shown in (a) of FIG. 20.
In FIG. 20, T1, T2, T3 and T4 of the output signals of the beam detector 1 represent incident times of the beam spots B1, B2, B3 and B4 to the light receiving device 101, and T4 and T5 represent times at which all beams B1, B2, B3 and B4 are irradiated to the photo-sensing device 101. T5, T6, T7 and T8 represent times at which the beams B1, B2, B3 and B4 sequentially move off the light receiving device 101. After T8, no light is irradiated to the light receiving device 101.
The output signal of the light receiving device 101 is amplified by an amplifier, not shown, and compared with a predetermined slice level set by a reference voltage circuit 12 (FIG. 17) by a comparator 11. The comparator 11 produces a detection signal which rises at the rise timing T1 of the beam B1 first applied to the light receiving device 101 and falls at the fall timing T8 of the beam B4 lastly moved off the light receiving device 101. The detection signal is applied to a timing signal generation circuit 13 which generates position signals corresponding to the beam spots B1-B4 at a proper timing. A detail of the timing signal generation circuit 13 is explained in conjunction with FIG. 21.
An output signal of the comparator 11 is used as a clear signal of a counter 1302 and the counter 1302 counts only when the detection signal is of H level. The counter 1302 counts a clock of an oscillation circuit 1301. A frequency of the oscillation circuit should be higher than an image recording clock frequency. As to the reduction of jitter due to the high frequency of the oscillation circuit 1301, reference is made to Japanese Laid-open Patent Application No. 51-89347, for example.
A timing signal (t) indicating the arrival of the first incident beam spot B1 at a predetermined position is generated by an R-S flip-flop (hereinafter referred to as FF). When the detection signal outputted from the comparator 11 is H level, the output of the R-S FF 1311 is H level. Simultaneously therewith, the counter 1302 starts to count the clock from the oscillation circuit 1301.
The output of the counter 1302 is multiple bit parallel output and it is inputted to data comparators 1304-1310. Compare inputs of the data comparators 1304-1310 have been preset by a group of switches 1303 such that a value M is inputted to the data comparator 1304 and a value N is inputted to the data comparator 1305. Thus, when the preset value coincides with the output of the counter 1302, the outputs of the data comparators 1304-1310 are H level which invert the outputs of the R-S FF 1311-1314.
For the timing signal (t), when the data comparator 1304 outputs a coincidence output by counting the count M of the counter 1302, the R-S FF 1311 is reset (FIG. 20). For a timing signal (u) indicating the arrival of the beam spot signal B2, the R-S FF is set by counting the count N1 of the counter 1302, and the R-S FF is reset when the counter 1302 counts N1+M (FIG. 20). Similarly, the R-S FF 1313 and the R-S FF 1314 are set by counting N2 and N3 by the counter 1302 and reset by the counts N2+M and N3+M, respectively.
For the timing signals (t)-(u), the rising timing thereof is important and the widths thereof may be of any value. The rising timing should coincide with a rise of a stepwise signal in the output signal of the beam detector 1. Thus, the rising timing of the timing signals (u)-(w) is adjusted by switching the preset values N1, N2 and N3 of the switches 1. When the beam spots B1-B4 have passed over the light receiving device 101 of the beam detector 1, the detection signal which is the output signal of the comparator 11 is L level and the counter 1302 is cleared and stops the count operation.
The timing signals (t)-(w) outputted from the timing signal generation circuit 13 are inputted to step down circuits 402-405. When the timing signals (t)-(w) are applied, the step down circuits 402-405 steps down the clock frequency from the oscillation circuit 401 by a factor of 1/P. In this manner, the step down circuits 402-405 generates the image clock signal. The oscillation circuit 401 may share the oscillation circuit 1301 provided in the timing signal generation circuit to be used by the timing signal generation circuit 13.
An image data controller 6 has information in a dot matrix form to form an image on the record medium and outputs record information for each main scan line to line buffers 701-704. Thus, the line buffers 701-704 has the record information for the scan lines. The line buffers 701-704 sequentially output the record information contained therein as the image clock signal is applied. The output record information is transferred to laser drive modulation circuits 801-804 to modulate lasers 9a, 9b, 9c and 9d arranged in the light source unit 9 to emit modulated laser beams ha, hb, hc and hd.
In such a prior art apparatus, since the detection signal is generated by detecting the rising timing of the first incident beam to the beam detector and the timing signal for the beam is generated by the detection signal for each elapse of the predetermined time period, the following problems are encountered.
(1) In the assembling process, the adjustment step to set the predetermined time period to generate the position detection signal for each beam is required, which leads to the increase of cost.
(2) When the timer is used to generate the timing signal, the precision of the timer determines the scan start position and hence a high precision timer is required. Thus, the cost reduction is difficult to attain.
(3) Since the clock frequencies of the oscillation circuits used for the timing signal generation circuit and the step down circuit are high, spurious noises are readily generated from wiring patterns on a printed circuit board.